This species is characterized by its rather robust oblanceolate leaves, lax pyramidal inflorescences with pink axes and numerous medium-sized flowers, tubular red and green corollas with the anthers partially exserted, and somewhat small obovoid capsules. The inflorescences were described as "secundiflorous" by Delprete (1999), meaning that the axes are held horizontally and the flowers are all pendulous. The corollas are red on the tube and greenish yellow or green on the lobes. The corollas are shortly angled to obtuse in bud, and at anthesis have cylindrical tubes 16-19 mm long and lobes 2-3.5 mm long. The fruits on the infructescences are less evidently secund.

Rustia rubra is similar to Rustia schunkeana, Rustia viridiflora, and Rustia thibuadioides.